The AD-600A Power Supply
This is the Power Supply PCB. Note the yellow line. Any parts above that line are at mains voltage potential and TOUCHING THEM WITH THE POWER PLUG IN THE WALL MAY KILL YOU so beware!
The power supply is a fairly conventional switchmode design based around the UC3842 controller chip. Most regulation of the Low Voltage side of the power supply is actually done on the High Voltage side by this controller chip. The only exception to this is A5V, -5V and 3.6V rails which have their own regulation parts. The output voltages from this board are +3.6, +5, +12, -5, -12, -24 and a + and - "F" voltage for the front panel fluorescent display.
Most of the problems with the power supply are due to high-voltage surges through the mains house supply either by other appliances, or lightning. The main casualties are the four diodes at the top (D1-4), the fuse and the main chopper FET (Q1). Luckily these parts can be replaced easily with common variety substitutes.
If you would like to get a copy of the power supply circuit you can download mine which I drew up. Remember that these circuits are a guide only and part values may differ from American 120V players.
Click HERE to get the circuits
COMMON PROBLEMS
If you have any contributions or questions with your power supply problems please email me and I shall include your findings in the list.
CAUTION
Before you proceed any further, please remember that working on power supplies can be dangerous and the voltages present CAN KILL YOU. So unless you have a sound knowledge of repairing switchmode power supplies please leave it to an expert. I will not be held responsible for your actions.
By far the most common problem is that you have recently had a close lightning strike and/or power surge and now your player is dead. The reason for this is that there are many parts on the PSU board that are directly connected to the mains supply and are only rated at maybe 50 volts higher than their usual operating condition. These parts are above that yellow line on the picture at the top of this page. When you get a power surge or spike these parts exceed their maximum working voltage and therefore they fail.
You will find that more often than not you have a blown fuse. Vary rare, but sometimes just replacing this fuse will fix the problem. BUT BE CAREFUL! Do not just stick any old fuse in, or bridge the fuse out with a piece of wire. Doing this is asking for trouble and will cause more serious problems like the board catching on fire and possibly burning your house down. The fuse plays an important safety function and should be replaced with EXACTLY the same type as the one which has blown.
More often than not the fuse has blown because of something else wrong. Your first replacement fuse will probably blow straight away which means you have a problem elsewhere on the psu board. The most common problem here is a shorted component and you can easily check this by placing an ohms meter across C2 or alternatively C15. If you read a low ohms (or a dead short) then check D1 to D4, D6 and Q1. These are the most likely causes for a fuse to keep blowing. Other parts which may cause this (but less likely) are C1, C2, C3, C4, and C15.
If however a new fuse does not blow but you still have no sign of life on any of the low voltage connectors on the psu board then you have something which is open circuit. If all the low voltage outputs are completely dead then the fault is most likely on the high voltage side of the transformer. Common parts to go open circuit are R3, R4, R6, R7, R9, R11, R12, D5, D6, D15 and Q1. Just a word of caution here: When checking high-ohms resistors like R3 and R7 and R12 it is often best to de-solder one leg clear of the board so you are checking that part only. Many times you will get a false reading if the part is left in circuit from another part "dragging down" the measurement. In my experience, high-ohms resistors like R3 and R7 in high-voltage applications do not mix and often go open circuit for no reason. You may also have U1 shutting down the power supply because it is sensing a short circuit on the Low Voltage side. This may be caused by a shorted part in any of the voltage rails. More on this below.
On the Low-Voltage side of faults, you can have many different problems which I will talk about seperately. For starters, you may have all but one voltage rail. This is where you must "sectionalise" the low-voltage circuit and work out what parts are related to particular voltage sections. OK, for example, you have all voltages but D12V. Working back from CN5 you can trace that the only parts related to D12V rail are R29, C23, L2, C22 and D12. As you can see only approx 10% of the whole low voltage parts are related to D12V.
However, you may strike a situation where all the voltage rails are low. Ruling out a regulation fault on the High Voltage side of the psu, we can assume that there must be a partial short somewhere on the Low Voltage side. This may take some finding, but you can be assisted by feeling if any parts are getting very hot. This is a dead giveaway that something is wrong.
Generally though a shorted part on the Low Voltage side will cause the UC3842 chip on the High Voltage side to sense a high-current drag and shut down the power supply. The only way to find this is to start checking all voltage rails with an ohm meter, looking for a low ohms (or short circuit). check between each voltage rail and GND in turn at the output connectors. If you find one rail which is shorted then work backwards towards T1 (the transformer) until you find the offending part.
And a contribution from T.S. who emailed me today. T.S. works with switchmode type power supplies and has a vaild point: "I have over 20 years on, in, and around electronics, so please pass on that Your "visitors" will want to replace the electrolytic caps in/on the power supply made by Nichicon! These caps are known to leak when exposed to heat for any length of time. When this happens, (Yep I said When!) they might just take the rest of the unit with them. I speak from experience on this! We are replacing the same ones, used in Our units for this same reason. The problem isn't a recent thing, I work on units back to 1983, and up to the present. I saw the picture on Your site and felt I should Warn all of You! All the best!" Thanks T.S. I have been told before that certain brands of electrolytic capacitors are not as good as others but I was never told which type. Electrolytic capacitors are those round can cylinder looking type of things you can see on the power supply board above This may well serve a warning to anyone with an already dead power supply and to the rest of us to check the brand of capacitors used in our own units. Keep those contributions on your faults/repairs coming.
That is all for now. If you have any further faults you wish me to discuss here please feel free to email me with the details.
